The present invention concerns a timepiece movement including a plate, a display device with hands, a gear train connecting drive means to said hands, said train including a drive wheel meshed at least indirectly with said drive means and secured to a cannon-pinion pivoting on a post mounted in a gear train bar, an hour wheel arranged around the cannon-pinion and resting on the plate, the drive wheel and the hour wheel extending respectively on either side of said plate.
This type of timepiece movement is common and has been known for a long time. However, it should be noted that the movements of the prior art have certain drawbacks, which the present invention can overcome.
FIG. 1 shows schematically a part of a movement with centre-seconds as disclosed in the prior art. FIG. 1 shows the relative positions of the hour display wheel sets. In particular, a seconds wheel set 101 can be seen, mounted so as to pivot in a jewel 102, which is itself driven into a gear train bar. The arbour 104 of the seconds wheel set is disposed in a central pipe 105, said central pipe 105 being driven through a plate 106. The seconds arbour 104 includes, in its median part, an annular raised portion 107 whose diameter is substantially equal to the inner diameter of central pipe 105. Said central pipe 105 fulfills thus the function of radial guide for the second arbour 104. Cannon-pinion 108 is rotatably mounted on said central pipe 105, the base of cannon-pinion 108 resting on the top of plate 106. It should be noted that central pipe 105 also fulfils the function of a radial guide for cannon-pinion 108. The pinion of cannon-pinion 108 meshes with a motion-work wheel 110, secured to a motion-work pinion 11 which meshes with an hour wheel 112. Said hour wheel 112 is rotatably mounted on cannon-pinion 108 which thus acts as its radial guide.
A first drawback of this type of movement is clear from FIG. 1 and concerns timepiece movement manufacturers, in particular when they deliver such movements to clients who then encase them in watch cases of their choice. Indeed, in a conventional manner, whereas all of movement gear train is enclosed between a plate and bars, the hour wheel is disposed on said plate, and thus outside the movement. Moreover, given that said hour wheel has to be free to rotate about the cannon-pinion, it is typically simply slipped onto said cannon-pinion and is thus free axially. Consequently, the hour wheel is capable of becoming unmeshed from the motion-work pinion during transport from the movement manufacturer to the client. More precisely, during the timepiece assembly process, the dial is generally pressed onto the movement. If, during this step, the hour wheel is not properly meshed with the motion-work pinion, the teeth of the hour wheel can be deformed because of pressure against against the motion-work pinion. In the case of inexpensive movements, the hour wheel is sometimes made of plastic material and its teeth are then likely to break when the dial is set in place, if the toothing of the hour wheel is not properly meshed with the motion-work pinion toothing. The client is obliged, prior to setting the dial in place, to check automatically or manually that the hour wheel is properly positioned, if he wishes to avoid risking any malfunction of the timepiece, following damage to the movement. This unavoidable checking step involves an increase in the duration and cost of manufacture of the finished product.
Likewise, the hour wheel may come entirely free of the cannon-pinion and get lost during transport of the movements to the encasing station, which also involves additional production costs.
A known solution of the prior art for overcoming this problem consists in adding a washer to the cannon-pinion pipe once the hour wheel is set in place, so as to lock the latter axially. However this solution has a drawback in that it requires the manufacture of an additional part, typically a washer, and the implementation of an additional step in the movement assembly process, in order to mount said washer.
The main object of the present invention is thus to overcome the drawbacks of the aforementioned prior art by providing a timepiece movement, allowing in particular the hour wheel to be kept meshed with the motion-work pinion during transport of the movements before the encasing operation.
The invention therefore concerns a timepiece movement of the aforementioned type, characterised in that the hour wheel and the cannon-pinion respectively include complementary means for holding the hour wheel axially on the cannon-pinion.
The structure according to the invention prevents the hour wheel coming free of the cannon-pinion during the transport step and thus being lost or damaged when the dial is set in place, while omitting the aforementioned checking step.
An additional advantage of the invention stems from the fact that the means for holding the hour wheel on the cannon-pinion pipe form an integral part of these two elements. Manufacture of the holding means thus does not necessitate implementation of an additional step, as such, in the movement manufacturing process.
According to a particular embodiment of the invention, said holding means are of the snap fitting type. They may include in particular a complementary bulge and groove, respectively arranged on the pipe of the hour wheel and on the cannon-pinion tube, for example.
An additional advantage of the present invention resides in the fact that said bulge is made by an over-thickness of material on the hour wheel pipe, said over-thickness being located substantially where the hour hand is subsequently driven in. Indeed, driving in a hand generally causes a plastic flow of material onto its support which may increase over time. This phenomenon may create play between the hand and its support, or even cause the hand to become detached from its support. Owing to the structure of the hour wheel pipe according to the invention, in particular owing to said over-thickness, the plastic flow of material on the tube is limited and the hour hand is held securely over time.
Another drawback of the movements of the prior art such as that shown in FIG. 1 concerns manufacturing tolerances to be taken into account for making the various parts forming the movement. More particularly here, the tolerances on the radial dimensions of the seconds wheel set 101, central pipe 105, cannon-pinion 108 and hour wheel 112 are of great importance. Indeed, these different parts have to be both free to rotate with respect to each other and act as a radial guide for each other. The drawback of such a structure is that uncertainties as to the radial dimensions of a part, directly linked to tolerances accepted by the manufacturer, will be added to radial uncertainties existing on the following part, and so on. Globally, the sum of all these uncertainties may lead to a finished product of mediocre manufacturing quality, because of a significant radial play of the arbour of the hour hand for example.
An obvious solution allowing this problem to be overcome consists in reducing the accepted manufacturing tolerances for the movement parts, which has the result of limiting radial uncertainties and thus the final play of the hour hand arbour. However, limiting tolerances, however slightly, causes significant increases in production costs, which is unsuitable for the manufacture of inexpensive movements.
Another object of the present invention is to overcome the aforementioned drawbacks of the prior art by providing a timepiece movement of acceptable manufacturing quality at a low cost price.
The invention thus also concerns a timepiece movement of the aforementioned type, characterised in that it does not include a central pipe and that an annular shoulder is provided on the movement plate acting as a radial guide for the hour wheel. Thus the global radial uncertainty is greatly reduced with respect to that of movements of the prior art insofar as a part, and thus its accompanying uncertainty, is omitted. The economic importance of the simple fact of omitting one part from a movement, should be recalled here, more particularly when it is an inexpensive movement manufactured in batches of at least several hundred thousand or several million.
Moreover, the various aforementioned uncertainties have been in part disassociated insofar as the cannon-pinion no longer fulfils the function of radial guide for the hour wheel. Consequently, the radial play existing as regards the hour wheel in no way depends upon the radial uncertainties inherent in the manufacture of the seconds wheel set and cannon-pinion. Thus, the radial play of the arbour of the hour hand can be suitably controlled, independently of the uncertainties existing as regards the seconds wheel set and cannon-pinion.